This invention relates to a method for removing the noise contained in a speech signal and for suppressing or reducing the noise therein.
In the field of portable telephone sets or speech recognition, it is felt to be necessary to suppress noise such as background noise or environmental noise contained in the collected speech signal for emphasizing its speech components.
As a technique for emphasizing the speech or reducing the noise, employing a conditional probability function for attenuation factor adjustment is disclosed in R. J. McAulay and M. L. Maplass, "Speech Enhancement Using a Soft-Decision noise Suppression Filter," in IEEE Trans. Acoust., Speech Signal Processing, Vol. 28, pp. 137 to 145, April 1980.
In the above noise-suppression technique, it is a frequent occurrence that unspontaneous sound tone or distorted speech is produced due to an inappropriate suppression filter or an operation based upon an inappropriate fixed signal-to-noise ratio (SNR). It is not desirable for the user to have to adjust the SNR, as one of the parameters of a noise suppression device, in actual operation for realizing an optimum performance. In addition, it is difficult with the conventional speech signal enhancement technique to eliminate the noise sufficiently without generating distortion in the speech signal that is susceptible to significant variation in the SNR in short time.
Such speech enhancement or noise reducing technique employs a technique of discriminating a noise domain by comparing the input power or level to a pre-set threshold value. However, if the time constant of the threshold value is increased with this technique for prohibiting the threshold value from tracking the speech, a changing noise level, especially an increasing noise level, cannot be followed appropriately, thus leading occasionally to mistaken discrimination.
For overcoming this drawback, the present inventors have proposed in JP Patent Application Hei-6-99869 (1994) a noise reducing method for reducing the noise in a speech signal.
With this noise reducing method for the speech signal, noise suppression is achieved by adaptively controlling a maximum likelihood filter configured for calculating a speech component based upon the SNR derived from the input speech signal and the speech presence probability. This method employs a signal corresponding to the input speech spectrum less the estimated noise spectrum in calculating the speech presence probability.
With this noise reducing method for the speech signal, since the maximum likelihood filter is adjusted to an optimum suppression filter depending upon the SNR of the input speech signal, sufficient noise reduction for the input speech signal may be achieved.
However, since complex and voluminous processing operations are required for calculating the speech presence probability, it has been desired to simplify the processing operations.